The problem of container deformation in response to pressure differences existing between the inside of a closed container and the ambient pressure is well known in the packaging industry. Such container deformation may be non-recoverable for certain container materials, like some plastics or metals. Thin-walled, partially flexible containers are particularly sensitive to the problem.
There are a number of possible factors which may lead to the existence of the pressure differences between the interior and the exterior of the container mentioned above. The content of the container may, for example, be chemically unstable or may be subject to reaction with gases which may exist in the head space of the container, or alternatively, in certain specific circumstances, may react with the container material itself. Any chemical reactions involving the liquid contents may lead to either production of gases, and hence to overpressure in the container, or to the absorption of any head space gases thereby causing underpressure in the container.
Pressure differences between the pressure inside the container and the ambient atmospheric pressure may also occur when the temperature during the filling and sealing of the container is significantly different from external temperature during shipment, transportation and storage. Another possibility of a pressure difference may be caused by a different ambient pressure at the filling of the container from another ambient pressure at a different geographical location.
The prior art has proposed several solutions using valve systems which avoid pressure differences between the interior and the exterior of the container. Proposed solutions also relate to various venting caps which allow pressure generated inside the container to be released by escape of gas. For example, FR-A-2 259 026, U.S. Pat. No. 4,136,796 and DE-A-2 509 258 disclose self-venting closures comprising a gas-permeable membrane covering an orifice to the exterior. Said membranes are made of a material which is impermeable to liquids, but permeable to gases. Therefore, containers may comprise apertures to release gas to the exterior without losing their leak-tightness. Another example is EP-A-593 840 which discloses containers for containing liquids which generate pressure, said container being made of a thermoplastic material comprising a network of micro-channels. This network of microchannels is permeable to gases, but not to liquids.
We found that should liquid product contact these membranes, or the extremity of micro-channels, said membranes may lose at least part of their gas-permeability. Indeed, liquid products which are viscous or which have some affinity for these membranes may not drain away from said membrane back into the container. In this manner, it may happen that the container loses venting capacity. This loss of venting capacity results in a pressure difference between the exterior and the inside of said container which may deform said container. The contact between said product and said membrane may be caused by splashes of said product onto said membrane as the filled container is agitated during shipment and transportation of the container. We found that the amount of splashes normally occurring during shipment and transportation are sufficient to completely interrupt the venting capacity of said container. Another means by which product may contact with the membrane is during an upside down storage of the container. We further found that other venting systems, like valves for example, may also suffer from a similar disadvantage.
We further found that an important parameter which influences the draining away of said product from said membrane is that the product which has contacted said membrane may undergo phase separation. Specifically, we found that for certain type of products draining may be improved when phase separation is enhanced. On the contrary, we further found that phase separation induced on other different products substantially reduces the draining away from said venting means, and consequently reduces venting capacity of said venting means. Therefore, phase separation of the splashed product through said membrane is an important parameter which determines the venting capacity of said venting means.
It is therefore an object of the present invention to provide a container (10) for a liquid product, or a cap (10) for such a container which allows venting of said product by a venting means (20), and allows control of the phase separation of said product which is in contact with said venting means.